Solar energy, radiant light and heat from the sun, is harnessed using a range of ever evolving
technologies such as solar heating, solar photovoltaic, solar thermal electricity,
solar architecture and artificial photosynthesis.
Solar energy in one form or another is the source of nearly all energy on the earth.
Humans, like all other animals and plants, rely on the sun for warmth and food. However,
people also harness the sun’s energy in many other different ways. For example, fossil
fuels, plant matter from a past geological age, is used for transportation and electricity
generation and is essentially just stored solar energy from millions of years ago.
Similarly, biomass converts the sun’s energy into a fuel, which can then be used for heat,
transport or electricity. Wind energy, used for hundreds of years to provide mechanical
energy or for transportation, uses air currents that are created by solar heated air and the
rotation of the earth. Today wind turbines convert wind power into electricity as well as
its traditional uses. Even hydroelectricity is derived from the sun. Hydropower depends
on the evaporation of water by the sun, and its subsequent return to the Earth as rain to
provide water in dams. Photovoltaic (often abbreviated as PV) is a simple and elegant
method of harnessing the sun’s energy. PV devices (solar cells) are unique in that they
directly convert the incident solar radiation into electricity, with no noise, pollution or
moving parts, making them robust, reliable and long lasting.
The Earth receives 174 petawatts (PW) of incoming solar radiation (insolation) at the
upper atmosphere. Approximately 30% is reflected back to space while the rest is
absorbed by clouds, oceans and land masses. The spectrum of solar light at the Earth’s
surface is mostly spread across the visible and near-infrared ranges with a small part in
the near-ultraviolet.
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Earth’s land surface, oceans and atmosphere absorb solar radiation, and this raises their
temperature. Warm air containing evaporated water from the oceans rises, causing
atmospheric circulation or convection. When the air reaches a high altitude, where the
temperature is low, water vapor condenses into clouds, which rain onto the Earth’s
surface, completing the water cycle. The latent heat of water
FIG (1) SOLAR RADIATION ON EARTH
Condensation amplifies convection, producing atmospheric phenomena such as wind,
cyclones and anti-cyclones. Sunlight absorbed by the oceans and land masses keeps the
surface at an average temperature of 14 ??C by photosynthesis green plants convert solar
energy into chemical energy, which produces food, wood and the biomass from which
fossil fuels are derived.
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The total solar energy absorbed by Earth’s atmosphere, oceans and land masses is
approximately 3,850,000 exajoules (EJ) per year. In 2002, this was more energy in one
hour than the world used in one year. Photosynthesis captures approximately 3,000 EJ
per year in biomass. The technical potential available from biomass is from 100′
300 EJ/year. The amount of solar energy reaching the surface of the planet is so vast that
in one year it is about twice as much as will ever be obtained from all of the Earth’s nonrenewable
resources of coal, oil, natural gas, and mined uranium combined, Solar energy
can be harnessed at different levels around the world, mostly depending on distance from
the equator.
2.2) HISTORY OF SOLAR CELL
Photo voltaics is the process of converting sunlight directly into electricity using
solar cells. Today it is a rapidly growing and increasingly important renewable alternative
to conventional fossil fuel electricity generation, but compared to other electricity
generating technologies, it is a relative newcomer, with the first practical photovoltaic
devices demonstrated in the 1950s. Research and development of photo voltaics received
its first major boost from the space industry in the 1960s which required a power supply
separate from "grid" power for satellite applications. These space solar cells were several
thousand times more expensive than they are today and the perceived need for an
electricity generation method apart from grid power was still a decade away, but solar
cells became an interesting scientific variation to the rapidly expanding silicon transistor
development with several potentially specialized niche markets. It took the oil crisis in
the 1970s to focus world attention on the desirability of alternate energy sources for
terrestrial use, which in turn promoted the investigation of photovoltaics as a means of
generating terrestrial power. Although the oil crisis proved short-lived and the financial
incentive to develop solar cells abated, solar cells had entered the arena as a power
generating technology. Their application and advantage to the "remote" power supply
area was quickly recognized and prompted the development of terrestrial photo voltaic
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industry. Small scale transportable applications (such as calculators and watches) were
utilised and remote power applications began to benefit from photovoltaic.
In the 1980s research into silicon solar cells paid off and solar cells began to increase
their efficiency. In 1985 silicon solar cells achieved the milestone of 20% efficiency.
Over the next decade, the photovoltaic industry experienced steady growth rates of
between 15% and 20%, largely promoted by the remote power supply market. The year
1997 saw a growth rate of 38% and today solar cells are recognized not only as a means
for providing power and increased quality of life to those who do not have grid access,
but they are also a means of significantly diminishing the impact of environmental
damage caused by conventional electricity generation in advanced industrial countries.
2.3) Properties of Light
The light that we see every day is only a fraction of the total energy emitted by the sun
incident on the earth. Sunlight is a form of "electromagnetic radiation" and the visible
light that we see is a small subset of the electromagnetic spectrum shown at the right.
The electromagnetic spectrum describes light as a wave which has a particular
wavelength. The description of light as a wave first gained acceptance in the early 1800’s
when experiments by Thomas Young, Fran??ois Arago, and Augustin Jean Fresnel
showed interference effects in light beams, indicating that light is made of waves. By the
late 1860’s light was viewed as part of the electromagnetic spectrum. However, in the late
1800’s a problem with the wave-based view of light became apparent when experiments
measuring the spectrum of wavelengths from heated objects could not be explained using
the wave-based equations of light. This discrepancy was resolved by the works of 1in
1900, and 2 in 1905. Planck proposed that the total energy of light is made up of
indistinguishable energy elements, or a quantum of energy. Einstein, while examining the
photoelectric effect (the release of electrons from certain metals and semiconductors
when struck by light), correctly distinguished the values of these quantum energy
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elements. For their work in this area Planck and Einstein won the Nobel Prize for physics
in 1918 and 1921, respectively and based on this work, light may be viewed as consisting
of "packets" or particles of energy, called photons.
2.4) SOLAR CELL
A solar cell is an electronic device which directly converts sunlight into electricity. Light
shining on the solar cell produces both a current and a voltage to generate electric power.
This process requires firstly, a material in which the absorption of light raises an electron
to a higher energy state, and secondly, the movement of this higher energy electron from
the solar cell into an external circuit. The electron then dissipates its energy in the
external circuit and returns to the solar cell. A variety of materials and processes can
potentially satisfy the requirements for photovoltaic energy conversion, but in practice
nearly all photovoltaic energy conversion uses semiconductor materials in the form of
a p-n junction.
FIG (2) SOLAR CELL STRUCTURE
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The basic steps in the operation of a solar cell are:
‘ the generation of light-generated carriers;
‘ the collection of the light-generated carries to generate a current;
‘ the generation of a large voltage across the solar cell; and
‘ The dissipation of power in the load and in parasitic resistances.
‘ The generation of current in a solar cell, known as the "light-generated current",
involves two key processes. The first process is the absorption of incident photons
to create electron-hole pairs. Electron-hole pairs will be generated in the solar cell
provided that the incident photon has energy greater than that of the band gap.
However, electrons (in the p-type material), and holes (in the n-type material) are
meta-stable and will only exist, on average, for a length of time equal to the
minority carrier lifetime before they recombine. If the carrier recombines, then the
light-generated electron-hole pair is lost and no current or power can be generated.
‘ A second process, the collection of these carriers by the p-n junction, prevents this
recombination by using a p-n junction to spatially separate the electron and the
hole. The carriers are separated by the action of the electric field existing at the pn
junction. If the light-generated minority carrier reaches the p-n junction, it is
swept across the junction by the electric field at the junction, where it is now a
majority carrier. If the emitter and base of the solar cell are connected together
(i.e., if the solar cell is short-circuited), the light-generated carriers flow through
the external circuit.
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2.5) The photovoltaic effect
The collection of light-generated carriers does not by itself give rise to power generation.
In order to generate power, a voltage must be generated as well as a current. Voltage is
generated in a solar cell by a process known as the "photovoltaic effect". The collection
of light-generated carriers by the p-n junction causes a movement of electrons to the ntype
side and holes to the p-type side of the junction. Under short circuit conditions, there
is no build up of charge, as the carriers exit the device as light-generated current.
However, if the light-generated carriers are prevented from leaving the solar cell, then the
collection of light-generated carriers causes an increase in the number of electrons on
the n-type side of the p-n junction and a similar increase in holes in the p-type material.
This separation of charge creates an electric field at the junction which is in opposition to
that already existing at the junction, thereby reducing the net electric field. Since the
electric field represents a barrier to the flow of the forward bias diffusion current, the
reduction of the electric field increases the diffusion current. A new equilibrium is
reached in which a voltage exists across the p-n junction. The current from the solar cell
is the difference between IL and the forward bias current. Under open circuit conditions,
the forward bias of the junction increases to a point where the light-generated current is
exactly balanced by the forward bias diffusion current, and the net current is zero. The
voltage required to cause these two currents to balance is called the "open-circuit
voltage". The following animation shows the carrier flows at short-circuit and opencircuit
conditions.
2.6) Short-Circuit Current
The short-circuit current is the current through the solar cell when the voltage across the
solar cell is zero (i.e., when the solar cell is short circuited). Usually written as ISC, the
short-circuit current is shown on the IV curve below.
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FIG (3) IV curve of a solar cell showing the short-circuit current.
The short-circuit current is due to the generation and collection of light-generated
carriers. For an ideal solar cell at most moderate resistive loss mechanisms, the shortcircuit
current and the light-generated current are identical. Therefore, the short-circuit
current is the largest current which may be drawn from the solar cell.
The short-circuit current depends on a number of factors which are described below:
‘ The area of the solar cell. To remove the dependence of the solar cell area, it is more
common to list the short-circuit current density (Jsc in mA/cm2) rather than the shortcircuit
current;
‘ The number of photons (i.e., the power of the incident light source). Isc from a solar
cell is directly dependant on the light intensity as discussed in Effect of Light
Intensity;
‘ The spectrum of the incident light. For most solar cell measurement, the spectrum is
standardized to the AM1.5 spectrum;
‘ The collection probability of the solar cell, which depends chiefly on the surface
passivation and the minority carrier lifetime in the base.
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2.7) Open-Circuit Voltage
The open-circuit voltage, VOC, is the maximum voltage available from a solar cell, and
this occurs at zero current. The open-circuit voltage corresponds to the amount of forward
bias on the solar cell due to the bias of the solar cell junction with the light-generated
current. The open-circuit voltage is shown on the IV curve below.
FIG (4) IV curve of a solar cell showing the open-circuit voltage.
2.8) Efficiency
The efficiency is the most commonly used parameter to compare the performance of one
solar cell to another. Efficiency is defined as the ratio of energy output from the solar cell
to input energy from the sun. In addition to reflecting the performance of the solar cell
itself, the efficiency depends on the spectrum and intensity of the incident sunlight
and the temperature of the solar cell. Therefore, conditions under which efficiency is
measured must be carefully controlled in order to compare the performance of one device
to another. Terrestrial solar cells are measured under AM1.5 conditions and at a
temperature of 25??C. Solar cells intended for space use are measured under AM0
conditions. Recent top efficiency solar cell results are given in the page Solar Cell
Efficiency Results.
The efficiency of a solar cell is determined as the fraction of incident power which is
converted to electricity and is defined as:
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Where Voc is the open-circuit voltage
where Isc is the short-circuit current; and
where FF is the fill factor
where ?? is the efficiency.
The input power for efficiency calculations is 1 kW/m2 or 100 mW/cm2. Thus the input
power for a 100 ?? 100 mm2 cell is 10 W and for a 156 ?? 156 mm2 cell is 24.3 W.